In my experience when discs tighten up if you rely on the locking pin, the use of a hammer will do one of two things, either break the pin, or break the ali casing around it.

Use the molegrips to stop this happening.

You should be able to use a small punch like i suggested at 45 degrees to the hole.

I had nearly a legal battle with Machine Mart when i took back a grinder as i couldnt get a disc off, and they suggested i had abused it !!!

It wont be a left hand thread.

Good luck.

JOhn